In an internal combustion engine, thermal energy is released when fuel is burned. This thermal energy is converted into mechanical energy. In a four stroke or four cycle engine, the combustion gases, for example air, either with a separate stream of fuel or premixed, are introduced from intake manifolds through intake valves into the combustion chambers. The combustion gas is compressed in the cylinder between the piston and the top of the cylinder and ignited. Ignition is by heat of compression in the case of a diesel engine, and by an electric spark plug in the case of a gasoline engine. The combustion of the fuel-air mixture pushes the piston down. The piston, acting through a connecting rod, imparts rotary motion to the crank shaft. The spent, burnt gases are then removed from the cylinder through the outlet valve and replaced by fresh combustion mixture so that a new cycle can begin. The energy required for effecting the exhaust stroke is provided by the fly-wheel.
In a four cycle engine the first stroke is an intake stroke where the inlet valve is open, and the descending piston, draws fresh combustion gases into the cylinder. The second stroke is a compression stroke where the valves, that is the intake valves and the exhaust valves, are closed and the rising piston compresses the combustion gas mixture. The compression ration is from about 5:1 to about 10:1 in the case of a gasoline engine, and from about 14:1 to about 30:1 in the case of a diesel engine. The third stroke is the power stroke. With the valves still closed the combustion gas mixture is ignited and the pressure of the burning gases forces the piston downward. The fourth stroke is the exhaust stroke, in which the exhaust valve is open and the rising piston discharges the spent gases from the cylinder.
Diesel engines differ from gasoline engines in that in a diesel engine air alone is initially injected into the cylinder and compressed to a very high ratio, for example, from 14 to 1 to about 30 to 1. The resulting compression heats the air to a temperature of from about 700.degree. C. to about 1000.degree. C. At the end of the compression stroke, when the air is at a high temperature, a measured quantity of diesel fuel is injected into the cylinder. The injected diesel fuel ignites spontaneously. Spontaneous ignition occurs in approximately 0.1 to 1 millisecond after injection of the diesel fuel. This occurs after the diesel fuel droplets have mixed intimately with the heated air in the combustion chamber and have been heated to their ignition temperature.
As a result of the high pressures and high temperatures encountered in diesel engines, powerful springs are necessary to keep the valves closed during the compression and combustion cycles, and to assure opening of only the intake valves during the inlet cycle and only the exhaust valves during the exhaust cycle.
Moreover, the high temperatures involved place high thermal stresses on the materials of construction of diesel engine internal components.
The opening of the valves is controlled by a cam shaft. An individual cam lifts a tappet, which lifts a push rod. The push rod rotates a pivoted rocker arm which translates the push rod action 180 degrees. The rocker arm action is transmitted to a crosshead, and from the crosshead to a valve stem. The cam shaft has irregularly shaped cams which force the tappet and push rod vertically upward. This pushes the one end of the rocker arm vertically upward and the other end vertically downward onto the valve crosshead. The valve crosshead then forces the valve stem down to open the valve against the force of the valve springs.
The crosshead is a component that transfers the motion of the rocker arm to two valves. The contact area where the the rocker arm contacts the crosshead piece is highly prone to wear. It has been proposed to place a ceramic wear pad on the valve crosshead piece between the valve crosshead piece and the rocker arm whereby to take up the wear. The ceramic wear pad is to be bonded to the crosshead piece. However, commercial ceramic to metal brazing alloys and solders cannot be used to bond the ceramic wear pad to the metallic crosshead piece. This is because the temperatures normally required for bonding the ceramic wear pad to the crosshead piece degrade the crosshead piece.